Field of the Invention
The present invention relates to a Lamb wave device that uses Lamb waves, and to a method of manufacturing same.
Description of Related Art
Surface acoustic wave (SAW) filters and film bulk acoustic resonators (FBAR) are used in mobile phones to transmit and receive radio waves. SAW filters are mainly used for a frequency range of 3 GHz or lower, for example, and FBARs are mainly used for a frequency range of 1.6 GHz or higher, for example. Because next generation mobile phones use radio waves in the 3.5 GHz band, filter devices used for such next generation mobile phones are required to be able to handle a higher frequency than conventional filters (2 GHz band). In order to meet such demands, Lamb wave devices have been developed.
Similar to surface acoustic waves, Lamb waves are excited by IDT (interdigital transducer) electrodes formed on a piezoelectric substrate. Lamb waves and SAW differ from each other in that the SAW is propagated on the substrate surface and the Lamb waves are propagated by repeatedly reflecting off of the upper surface and the lower surface of the substrate. In Lamb wave devices, the thinner the piezoelectric substrate is, the faster the propagation speed becomes. For example, when the thickness of the piezoelectric substrate is 1 μm or less, the propagation speed of Lamb waves is 6000 m/s or higher, while the propagation speed of SAW is about 4000 m/s.
In Lamb wave devices, the thickness of the piezoelectric substrate largely affects the propagation speed. A change in the thickness of the piezoelectric substrate causes the propagation speed to change, and as a result, a resonance frequency changes. Therefore, in order to obtain desired resonance characteristics, it is necessary to control the thickness of the piezoelectric substrate with a higher degree of accuracy in manufacturing a Lamb wave resonator.
Patent Document 1 below, for example, describes a Lamb wave high frequency device that includes IDT electrodes formed on one surface of the piezoelectric substrate, and an adjustment film formed on the other surface of the piezoelectric substrate. The adjustment film is provided to adjust the resonance frequency. Patent Document 2 below describes a technique of selecting crystal orientations of a piezoelectric thin film so as to sufficiently slow down the etch rate, to hydrogen fluoride, of a surface of the piezoelectric thin film on the side facing a supporting member, thereby suppressing a change in thickness as a result of the piezoelectric thin film being dissolved.